Fabrication of semiconductor structures

ABSTRACT

The invention relates to a method for fabricating a semiconductor structure. The method comprises fabricating a photonic crystal structure of a first material, in particular a first semiconductor material and selectively removing the first material within a predefined part of the photonic crystal structure. The method further comprises replacing the first material within the predefined part of the photonic crystal structure with one or more second materials by selective epitaxy. The one or more second materials may be in particular semiconductor materials. The invention further relates to devices obtainable by such a method.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to a method forfabricating semiconductor structures, in particular for opticalapplications such as laser applications. Embodiments of the inventionfurther relate to semiconductor devices that may be fabricated with sucha method, in particular a resonator or more specifically a laser.

Optical interconnects for next-generation computing requiresemiconductor light sources. Photonic crystal nanocavities are idealplatforms for such semiconductor light sources as they may providestrong light-matter interaction, high Q/V ratios, threshold-less laserbehavior and high-speed modulation rates.

A challenge in photonic cavity-based devices is electrical actuation,i.e. to inject carriers efficiently to the active region withoutspoiling the optical cavity.

If the photonic cavity is implemented in the active material, excessiveabsorption losses may occur, because light is absorbed outside of thecavity.

On the other hand, a precise placement of the active material within thecentral part of the cavity remains a difficult challenge.

SUMMARY

According to a first aspect, the invention is embodied as a method forfabricating a semiconductor structure. The method comprises fabricatinga photonic crystal structure of a first material, in particular a firstsemiconductor material, and selectively removing the first materialwithin a predefined part of the photonic crystal structure. The methodfurther comprises replacing the first material within the predefinedpart of the photonic crystal structure with one or more second materialsby selective epitaxy. The one or more second materials may be inparticular semiconductor materials.

Such methods according to embodiments of the invention provide anefficient way to fabricate photonic crystal structures comprising twodifferent materials in an efficient manner.

According to embodiments, the properties of the first and secondmaterial may be improved or optimized independently to improve theefficiency of the device.

According to an embodiment, the predefined part of the photonic crystalstructure is a central part of the photonic crystal structure. Accordingto such an embodiment the second material can be placed in the centralpart of the photonic crystal structure. This may allow to confine thecarriers effectively in the central part or the center of the photoniccrystal structure, thereby improving the device performance.

While the first material may be in particular silicon, the secondmaterial may be in particular an optically active material. This mayallow e.g. to use current photonic crystal cavity designs having asuitable or compatible geometry, in particular Si-photonic designs, andreplace the silicon by the optically active material, e.g. by a groupIII-V material. Embodiments of the invention allow in particular toplace the optically active material as gain material in the parts of thephotonic crystal structure where it boosts the efficiency.

According to an embodiment, fabricating the photonic crystal structurecomprises providing a wafer comprising a layer of the first material andpatterning the layer of the first material, thereby fabricating thephotonic crystal structure of the first material. This allows anefficient fabrication.

According to an embodiment, the wafer is a silicon-on-insulator wafercomprising a silicon layer on an insulating layer and the method furthercomprises patterning the silicon layer, thereby fabricating the photoniccrystal structure comprising silicon as the first material. This allowsan efficient fabrication.

According to an embodiment, replacing the first material with the one ormore second materials comprises growing the one or more second materialsin a lateral direction of the wafer. This allows advantageous devicedesigns.

According to an embodiment, the method further comprises growing the oneor more second materials with a predefined doping profile in the lateraldirection of the wafer. This allows further advantageous device designs.In particular, methods according to embodiments of the invention mayallow epitaxial in-plane lateral doping, i.e. the fabrication of p-i-nstructures which are arranged parallel to the substrate of the wafer.

According to an embodiment, the method further comprises growing twodifferent second materials in the lateral direction of the wafer,thereby forming a lateral heterojunction. According to such anembodiment, in a first step a first one of the second materials, e.g. afirst III-V material such as AlGaAs or InP is epitaxially grown. And insecond step, another one of the second materials, e.g. another III-Vmaterial such as InGaAs is epitaxially grown, thereby fabricating thelateral heterojunction.

According to an embodiment, the method further comprises providingelectrical contacts to the one or more second materials. The provisionof such electrical contacts may be facilitated according to embodimentsin particular for the embodiments having lateral doping profiles.

According to embodiments, several different material combinations of thefirst material and a plurality of second materials may be implemented onthe same wafer by repeating growth runs. This may be possible due tolocal integration of the gain material.

According to an embodiment, selectively removing the first materialwithin the predefined part of the photonic crystal structure comprisesencapsulating the photonic crystal structure of the first material witha third material, in particular with an oxide material. Further stepsinclude selectively removing a part of the third material in thepredefined part of the photonic crystal structure to provide a window tothe first material and selectively removing a part of the first materialthrough the window. This creates a template structure of the thirdmaterial. The template structure may also be denoted as cavitystructure. In order to facilitate a growth of the one or more secondmaterials in the template structure, a remaining part of the firstmaterial forms a seed structure for the one or more second materials.

According to an embodiment, selectively removing the part of the firstmaterial through the window comprises performing a selective etching ofthe first material. In other words, the etching is performed such thatonly the first material is etched, but not the third material of thetemplate structure.

According to an embodiment, replacing the first material with the one ormore second materials comprises growing the one or more second materialswithin the template structure of the third material from the seedstructure. This is an efficient and precise way to arrange the secondmaterial within the photonic crystal structure.

According to an embodiment, the method further comprises removing theseed structure of the first material after the growing of the one ormore second materials within the template structure. This facilitates inparticular the electrical contacting of the second material(s).

According to an embodiment, the growing of the one or more secondmaterials is performed by one metal organic chemical vapor deposition(MOCVD), atmospheric pressure CVD, low or reduced pressure CVD,ultra-high vacuum CVD, molecular beam epitaxy (MBE), atomic layerdeposition (ALD) or hydride vapor phase epitaxy.

According to an embodiment of a further aspect of the invention, asemiconductor device obtainable by a method according to the firstaspect is provided.

According to an embodiment of a further aspect of the invention, asemiconductor device is provided which comprises a semiconductorsubstrate, an insulating layer on the semiconductor substrate and aphotonic crystal structure on the insulating layer. The photonic crystalstructure comprises a first material in an outer part and one or moresecond materials in a central part or area of the photonic crystalstructure, wherein the one or more second materials are epitaxiallygrown semiconductor materials forming a gain structure which extends ina lateral direction of the substrate.

According to an embodiment, the photonic crystal structure is1-dimensional, 2-dimensional or 3-dimensional photonic crystal lattice.

According to a further embodiment, the semiconductor device may beembodied as an optical resonator or laser. The gain structure maycomprise according to embodiments one or more quantum wells.

The steps of the different aspects of the invention may be performed indifferent orders as appropriate. Furthermore, the steps may also becombined as appropriate, i.e. that e.g. two or more steps may beperformed together.

Advantages of the features of one aspect of the invention may apply tocorresponding features of another aspect of the invention.

Embodiments of the invention will be described in more detail below, byway of illustrative and non-limiting examples, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a 3-dimensional view of an initial structure comprising alayer of a first material and FIG. 2a shows a corresponding top view;

FIG. 1b shows a 3-dimensional view comprising a photonic crystalstructure that has been formed from the layer of the first material bypatterning and FIG. 2b shows a corresponding top view;

FIG. 1c shows a 3-dimensional view of a structure after encapsulatingthe photonic crystal structure with a third material, e.g. an oxide andFIG. 2c shows a corresponding top view;

FIG. 1d shows a 3-dimensional view of the structure after a window hasbeen formed in the oxide and FIG. 2d shows a corresponding top view;

FIG. 1e shows a 3-dimensional view of the structure after a part of thefirst material has been selectively removed through the window and FIG.2e shows a corresponding top view;

FIG. if shows a 3-dimensional view after a second material has beengrown from a seed structure of the first material and FIG. 2f shows acorresponding top view; and

FIG. 1g shows a 3-dimensional view of the structure after the secondmaterial has been electrically contacted and FIG. 2g a corresponding topview;

FIG. 3 shows an exemplary top view of a semiconductor device accordingto an embodiment of the invention;

FIG. 4 is a scanning electron microscope image showing an exemplarydevice structure which has been fabricated with a method according to anembodiment of the invention; and

FIG. 5 shows a flow chart of method steps of a method for fabricating asemiconductor structure according to embodiments of the invention.

DETAILED DESCRIPTION

FIGS. 1a-1g show enlarged 3-dimensional views of initial, intermediateand final structures formed during the stages of fabrication methodsaccording to embodiments of the invention. FIGS. 2a-2g showcorresponding enlarged top views of the structures corresponding to theFIGS. 1a -1 g.

In any or all of the figures the dimensions may not be drawn to scaleand may be shown in a simplified and schematic way to illustrate thefeatures and principles of embodiments of the invention.

The term “on” and “above” are used in this context, as is customary, toindicate orientation or relative position in a vertical or orthogonaldirection to the surface of the substrate, in particular in a verticalz-direction.

The terms “lateral” or “laterally” are used in this context, as iscustomary, to indicate orientation generally parallel to the plane ofthe substrate, as opposed to generally vertically, or outwardly, fromthe substrate surface.

The term “arranged on the semiconductor substrate” shall be understoodin a broad sense and shall include in particular embodiments accordingto which an intermediate layer, e.g. an insulating layer, is arrangedbetween the substrate and the photonic crystal structure. Hence the term“arranged on the substrate” shall include the meaning arranged “abovethe substrate”.

FIG. 1a illustrates a 3-dimensional view of an initial structure 100 andFIG. 2a a corresponding top view. The initial structure 100 comprises asubstrate 110. The substrate 110, illustrated by diagonal upwardstripes, comprises a semiconductor material and may be e.g. a bulksemiconductor substrate. The substrate 110 may be embodied as acrystalline semiconductor or a compound semiconductor wafer of a largediameter. The substrate may comprise, for example, a material from groupIV of the periodic table as semiconductor material. Materials of groupIV include, for example, silicon, germanium, mixed silicon andgermanium, mixed silicon and carbon, mixed silicon germanium and carbonand the like. For example, the substrate 110 may be a crystallinesilicon wafer that is used in the semiconductor industry.

The structure 100 further comprises an insulating layer 111 on thesubstrate 110, illustrated by diagonal downward stripes. The insulatinglayer 111 may be embodied e.g. as a dielectric layer. The insulatinglayer 111 can be formed by known methods, as for example thermaloxidation, chemical vapor deposition (CVD), plasma enhanced CVD (PECVD),atomic layer deposition, chemical solution deposition, MOCVD,evaporation, sputtering and other deposition processes. Examples of suchdielectric material include, but are not limited to: SiO2, Si3N4, Al2O3,AlON, Ta2O5, TiO2, La2O3, SrTiO3, LaAlO3, ZrO2, Y2O3, Gd2O3, MgO, MgNO,Hf-based materials and combinations including multilayers thereof.

The structure 100 further comprises a layer 112 of a first materialembodied as semiconductor material on the insulating layer 111. Thefirst material may be in particular silicon. The layer 112 of the firstmaterial is illustrated with a dotted pattern of 30%.

The thicknesses of the substrate 110, the insulating layer 111 and thelayer 112 can be any suitable thicknesses.

The structure 100 may be in particular embodied as asilicon-on-insulator wafer.

FIG. 1b illustrates a 3-dimensional view of a structure 101 and FIG. 2ba corresponding top view. The structure 101 comprises a photonic crystalstructure 113 of the first material, e.g. of silicon. The structure 101has been formed by patterning the layer 112 of the first material. Thismay be performed e.g. by lithography and subsequent etching. Accordingto an embodiment, the patterning of the layer 112 may be performed by anetching based on HBr chemistry. The photonic crystal structure 113comprises a plurality of rods 114 of the first material. The rods 114are serially arranged next to each other with a predefined regulardistance d. The rods extend in a lateral direction of the substrate 110,more particularly parallel to the x-y-plane of the substrate 110. FIG.1c illustrates a 3-dimensional view of a structure 102 and FIG. 2c acorresponding top view. The structure 102 comprises an encapsulationlayer 115 which has encapsulated the photonic crystal structure 113 ofthe first material. The encapsulation layer 115 comprises a thirdmaterial, in particular a dielectric material, in particular an oxide.The encapsulation layer 115 will be used as template structure as willbe described further below. The encapsulation layer 115 completelycovers the rods 114. For ease of illustration, the encapsulation layer115 is shown in a transparent manner in the FIGS. 1c to 1 h.

FIG. 1d illustrates a 3-dimensional view of a structure 103 and FIG. 2da corresponding top view. The structure 103 comprises a window 116 whichhas been formed by selectively removing a part of the third material ofthe encapsulation layer 115 in a predefined part 120, shown in FIG. 2d ,of the photonic crystal structure 113. The window 116 establishes awindow to the first material of the photonic crystal structure 113. Inother words, the window 116 provides an opening to the photonic crystalstructure 113 within the predefined part 120. The predefined part 120 isarranged in a central part of the photonic crystal structure 113. Moreparticularly, the predefined part 120 is arranged symmetrically to asymmetry axis 121, shown in FIG. 2d , of the photonic crystal structure.The window 116 may be fabricated e.g. by lithography and etching. Thecentral part 120 of the photonic crystal structure is surrounded by orenclosed by outer parts 122, shown in FIG. 2d , of the photonic crystalstructure 113.

It should be noted that the rods 114 may generally have any desiredshape and geometry as suitable for the respective application. The rods114 may also be denoted as bars. According to embodiments, the geometryof the rods 114 in the outer parts 122 may be different from thegeometry of the rods 114 in the central part 120. As an example, therods 114 in the outer parts 122 may have a different length and widththan the rods 114 in the central part 120.

FIG. 1e illustrates a 3-dimensional view of a structure 104 and FIG. 2ea corresponding top view. In the structure 104, a part of the firstmaterial of the rods 114 in the central part 120 has been removedthrough the window 116. The rods 114 have created a hollow cavity ortemplate structure 117 within the third material. However, the rods 114have not been removed completely, but a remaining part of the rods 114form a seed structure 118 for a subsequent growth of one or more secondmaterials in the template structure 117. The selective removal of thefirst material through the window 116 may be performed by a selectiveetching of the first material, i.e. an etching that etches the firstmaterial, but not the third material. The suitable etching technique maydepend on the respective first and third material. In general, theetching technique is chosen to be such that it only etches the firstmaterial, but not the third material of the encapsulation layer. Theselective removal may be performed in particular by a dry or wet etchingof the first material.

FIG. 1f illustrates a 3-dimensional view of a structure 105 and FIG. 2fa corresponding top view. In the structure 105, the removed firstmaterial has been replaced within the predefined part 120 of thephotonic crystal structure 113 with one or more second materials byselective epitaxy. More particularly, the one or more second materialshave been grown within the template structure 117 of the third materialfrom the seed structure 118. The growing of the one or more secondmaterials has been performed in a lateral direction of the substrate110. The one or more second materials form rods 119 of the secondmaterial that have replaced the former rods 114 of the first materialwithin the central part 120 of the photonic crystal structure 113. Therods 119 of the second material are illustrated with a dotted pattern of70%.

The growing of the rods 119 of the second materials may be performede.g. by metal organic chemical vapor deposition (MOCVD), atmosphericpressure CVD, low or reduced pressure CVD, ultra-high vacuum CVD,molecular beam epitaxy (MBE), atomic layer deposition (ALD) or hydridevapor phase epitaxy.

The second materials of the rods 119 may be in particular opticallyactive materials. The second materials may be e.g. InP, InGaAs, AlGaAs,GaAs, GaN, InGaN, AlGaN, any other ternary or quaternary alloys thereof,group II-VI semiconductors or group IV semiconductors.

In general, the versatility of methods according to embodiments of theinvention may allow any combination of group III-V semiconductormaterials in the template structure 117, including embedded quantumwells, quantum dots, quantum wires, doped or intrinsic semiconductorlayers as well as heterojunctions.

FIG. 1g illustrates a 3-dimensional view of a structure 107 and FIG. 2ga corresponding top view. In the structure 106, electrical contacts 130have been provided which contact the one or more second materials of therods 119. The rods 119 are illustrated with a wave pattern.

According to further embodiments, depending on the respectiveapplication and post processing, there may be an additional step ofremoving the seed structure 118 of the first material before the step ofproviding the electrical contacts.

The rods 114 (as mentioned above) and accordingly the rods 119 maygenerally have any desired shape and geometry as suitable for therespective application. Hence according to embodiments, the geometry ofthe structure of the first material and the geometry of the structure ofthe second material may be different. As an example, the rods 119 of thesecond material may have a different length and width than the rods 114of the first material.

FIG. 3 shows an exemplary top view of a semiconductor device 300according to an embodiment of the invention. It comprises a photoniccrystal structure 313 which is arranged on an insulating layer 311 ofe.g. an SOI-wafer. The semiconductor device 300 may be fabricated withmethods according to embodiments of the invention as explained above.

The photonic crystal structure 313 comprises a plurality of rods 314 ofSi which are arranged in outer areas 322 of the photonic crystalstructure 313 and a plurality of rods 319 of one or more secondmaterials, in particular group III-V materials, in a central area 320 ofthe photonic crystal structure 313.

The plurality of rods 319 in the central area 320 form a gain structure.The rods 319 are embodied as p-i-n structures. The rods 319 have beenepitaxially grown and extend in a lateral direction of the substrate andthe photonic crystal structure, more particularly in an y-direction ofan x-y-plane. The x-y-plane is arranged in parallel to an underlyingsubstrate (not shown in FIG. 3). Accordingly, the rods 319 have anexemplary p-i-n doping profile. The rods 319 are contacted laterally byelectrical contacts 330, which are also denoted by “c” in FIG. 3. Thephotonic crystal structure 313 is embodied as 1-dimensional photoniccrystal lattice and establishes a photonic mirror with a gain structurein the central part or in other words central area 320.

Hence the embodied gain structure may include a doping profile whichforms a p-i-n-structure. This may facilitate electrical pumping. Ap-i-n-structure is a structure having an intrinsic region arrangedbetween a p-doped region and a n-doped region.

In this respect, doping shall be understood as the intentionalintroduction of impurities into an intrinsic semiconductor for thepurpose of modulating its electrical and optical and structuralproperties. Doping a semiconductor introduces allowed energy stateswithin the band gap, but very close to the energy band that correspondsto the dopant type. Positive or p-type doping introduces free holes inthe valence band, whereas negative or n-type doping introduces freeelectrons within the conduction band.

The introduction of dopants has the effect of shifting the energy bandsrelative to the Fermi level. In an n-type semiconductor the Fermi levelis close to the conductance band, or within the conductance band in adegenerate n-type semiconductor. For p-type the Fermi level is close tothe or within the Valance band. Doping densities in typically dopedsemiconductors range from 5×10¹⁸ cm⁻³ to 10²⁰ cm⁻³, depending on thematerial and density of states. Whereas semiconductors are rarelyperfectly intrinsic, intrinsic in the electrical sense means that theyare not conductive. Typically, the doping level is around 10¹⁵-10¹⁶cm⁻³.

The p-i-n structure may be grown in the template structure formed by theencapsulating oxide as follows:

In a first sub-step a n-doped semiconductor layer 351 of the secondsemiconductor material has been grown in the template structure. In asecond sub-step, an intrinsic layer 352 of the second semiconductormaterial has been grown. And in a third sub-step a p-doped semiconductorlayer 353 of the second semiconductor material has been grown. Thesemiconductor layers 351, 352 and 353 collectively form the gainstructure of the photonic crystal structure 313.

According to other embodiments a plurality of quantum wells, may begrown in the central part of the photonic crystal structure by growingsequentially in the template structure in an alternating way a pluralityof semiconductor layers of different semiconductor materials. Thedifferent semiconductor materials may have a different bandgap tofacilitate the formation of the quantum wells.

FIG. 4 is a scanning electron microscope image showing an exemplarydevice structure 400 which has been fabricated with a method accordingto an embodiment of the invention. The device structure 400 comprises aphotonic crystal structure 413 encompassing a plurality of Si-rods 414in outer areas 422 of the photonic crystal structure 413 and a pluralityof InP-rods 419 in a central area 420 of the photonic crystal structure413. The plurality of InP-rods 419 have been fabricated by replacingSi-rods in the central area 420 of the photonic crystal structure 413.

As an oxide-layer is covering the photonic crystal structure 413, therods 414 and 419 can only be seen in a shadowed way.

FIG. 5 shows a flow chart of method steps of a method for fabricating asemiconductor structure according to embodiments of the invention.

At a step 510, a wafer comprising a layer of the first material, inparticular a silicon-on-insulator wafer, is provided.

At a step 520, the layer of the first material is patterned, e.g. bylithography and etching, thereby forming a photonic crystal structure.

At a step 530, the photonic crystal structure of the first material isencapsulated with a third material, in particular an oxide.

At a step 540, a part of the third material is selectively removed inthe central part of the photonic crystal structure. This provides awindow or in other words an opening through the oxide to the firstmaterial.

At a step 550, the first material is partly and selectively removedthrough the window within the central part or in other words centralarea of the photonic crystal structure. This creates a hollow templatestructure or cavity structure of the third material. However, the firstmaterial is not removed completely, but performed such that a part ofthe first material remains in the template structure and provides a seedstructure for a subsequent growth of the one or more second materials.

At a step 560, the one or more second materials are grown from the seedof the first material within the template structure by selectiveepitaxy. As a result, the first material has been replaced within thecentral part of the photonic crystal structure with one or more of thesecond materials.

At a step 570, the seed structure of the first material may be removed,e.g. by etching. Whether this step is useful depends on the respectivedesign of the semiconductor structure and the respectivepost-processing.

At a step 580, electrical contacts are provided to the structure of thesecond materials that has been formed within the template structure ofthe third material.

It should be noted that the step 580 may be followed by furtherprocessing steps as appropriate to derive at a final device structure asdesired.

While illustrative examples are given above, it will be appreciated thatthe basic fabrication steps described above can be used to producesemiconductor structures of other materials, shapes and sizes. Materialsand processing techniques can be selected as appropriate for a givenembodiment, and suitable choices will be readily apparent to thoseskilled in the art.

While particular examples have been described above, numerous otherembodiments can be envisaged. The seed surfaces for growing thesemiconductor structures may be preferably crystalline seed surfaces,but may according to other embodiments also be provided by amorphoussurfaces. If the seed has a well-defined crystalline orientation and ifthe crystal structure of the seed is a reasonable match to that of thegrowing crystal (for example a III-V compound semiconductor), thegrowing crystal can adapt this orientation. If the seed is amorphous orhas an undefined crystal orientation, the growing crystal will be singlecrystalline, but its crystal orientation will be random.

The disclosed semiconductor structures and circuits can be part of asemiconductor chip. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip maybe integrated with other chips, discrete circuit elements, and/or othersignal processing devices as part of either an intermediate product,such as a motherboard, or an end product. The end product can be anyproduct that includes integrated circuit chips.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

As used herein, the articles “a” and “an” preceding an element orcomponent are intended to be nonrestrictive regarding the number ofinstances (i.e., occurrences) of the element or component. Therefore,“a” or “an” should be read to include one or at least one, and thesingular word form of the element or component also includes the pluralunless the number is obviously meant to be singular.

As used herein, the term “quantum well” is a non-limiting term and notintended to refer to only quantum well embodiments, but may encompassall possible quantum emitting systems like quantum dots and quantumwires.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method for fabricating a semiconductorstructure, the method comprising: fabricating a photonic crystalstructure of a first material; selectively removing the first materialwithin a predefined part of the photonic crystal structure; andreplacing the first material within the predefined part of the photoniccrystal structure with one or more second materials by selectiveepitaxy.
 2. The method as claimed in claim 1, wherein the predefinedpart of the photonic crystal structure is a central part of the photoniccrystal structure.
 3. The method as claimed in claim 1, whereinfabricating the photonic crystal structure comprises: providing a wafercomprising a layer of the first material; and patterning the layer ofthe first material, thereby fabricating the photonic crystal structureof the first material.
 4. The method as claimed in claim 3, wherein thewafer is a silicon-on-insulator wafer comprising a silicon layer on aninsulating layer.
 5. The method as claimed in claim 3, wherein replacingthe first material with the one or more second materials comprisesgrowing the one or more second materials in a lateral direction of thewafer.
 6. The method as claimed in claim 5, further comprising growingthe one or more second materials with a predefined doping profile in thelateral direction of the wafer.
 7. The method as claimed in claim 5,further comprising growing two different second materials in the lateraldirection of the wafer, thereby forming one or more lateralheterojunction.
 8. The method as claimed in claim 1, wherein selectivelyremoving the first material within the predefined part of the photoniccrystal structure comprises: encapsulating the photonic crystalstructure of the first material with a third material; selectivelyremoving a part of the third material in the predefined part of thephotonic crystal structure to provide a window to the first material;and selectively removing a part of the first material through thewindow, thereby creating a template structure of the third material,wherein a remaining part of the first material forms a seed structurefor the one or more second materials.
 9. The method as claimed in claim8, wherein selectively removing the predefined part of the firstmaterial through the window comprises performing a selective etching ofthe first material.
 10. The method as claimed in claim 8, whereinreplacing the first material with the one or more second materialscomprises growing the one or more second materials within the templatestructure of the third material from the seed structure.
 11. The methodas claimed in claim 1, wherein the photonic crystal structure of thefirst material comprises a plurality of rods of the first material. 12.The method as claimed in claim 8, further comprising removing the seedstructure of the first material after the growing of the one or moresecond materials within the template structure.
 13. The method asclaimed in claim 1, further comprising providing electrical contacts tothe one or more second materials.
 14. The method as claimed in claims inclaim 1, wherein patterning the first material comprises performing anetching based on HBr chemistry.
 15. The method as claimed in claim 1,wherein the growing of the one or more second materials is performed byone of: metal organic chemical vapor deposition (MOCVD); atmosphericpressure CVD; low or reduced pressure CVD; ultra-high vacuum CVD;molecular beam epitaxy (MBE); atomic layer deposition (ALD) and hydridevapor phase epitaxy.
 16. The method as claimed in claim 1, wherein theone or more second materials are optically active materials.
 17. Themethod as claimed in claim 1, wherein the one or more second materialsare selected from the group consisting of: InP; InGaAs; AlGaAs; GaAs;GaN; InGaN; AlGaN; a ternary or a quaternary alloy thereof; group II-VIsemiconductors and group IV semiconductors.
 18. The method as claimed inclaim 1, wherein the first material is silicon.
 19. The method asclaimed in claim 1, wherein the third material from the seed structureis a dielectric material, in particular an oxide.
 20. A semiconductordevice obtainable by the method according to claim
 1. 21. Asemiconductor device, comprising: a semiconductor substrate; aninsulating layer on the semiconductor substrate; and a photonic crystalstructure on the insulating layer, the photonic crystal structurecomprising a first material in an outer part and one or more secondmaterials in a central part of the photonic crystal structure, whereinthe one or more second materials are epitaxially grown semiconductormaterials forming a gain structure which extends in a lateral directionof the substrate.
 22. The semiconductor device according to claim 21,wherein the first material is silicon and the one or more secondmaterials are selected from the group consisting of: InP; InGaAs;AlGaAs; GaAs; GaN; InGaN; AlGaN; a ternary or a quaternary alloythereof; group II-VI semiconductors and group IV semiconductors.
 23. Thesemiconductor device according to claim 21, wherein the one or moresecond materials comprise a predefined doping profile in the lateraldirection of the substrate.
 24. The semiconductor device according toclaim 21, wherein the central part comprises two differentsemiconductors materials forming a heterojunction in the lateraldirection of the substrate.
 25. The semiconductor device according toclaim 21, wherein the photonic crystal structure is 1-dimensional,2-dimensional, or 3-dimensional photonic crystal lattice.